Electronic devices such as integrated circuits are relatively fragile and are easily damaged by mechanical impacts and some environmental effects such as corrosive fluids. To protect the electronic device during service, it is placed within a package structure. The package structure includes a housing that protects the electronic device, conducts heat away from the electronic device, and includes electrical feedthroughs to the electronic device.
The package structure housing is made of materials and platings selected in view of the operating requirements of the electronic device. Hydrogen is introduced into the package structure from several sources. The sources include base metal used for the construction of the device, such as KOVAR® and other iron based metals which have low thermal expansion coefficients. Attachment of electrical leads, which typically are brazed in place under a reducing atmosphere is another source of hydrogen. Plating finishes which are typically applied over the cover and braze alloy, such as the adhesion layer disclosed in U.S. Pat. No. 5,543,364 that are used to secure prior art venting packages and glue or braze materials used to seal the lid or cover to the base of the package, also discussed in U.S. Pat. No. 5,543,364, also undesirably devolve hydrogen into the package. Yet another source of hydrogen is any residual moisture that may remain in the package if it is hermetically sealed. Many of these materials and platings emit hydrogen gas gradually during service, so that the partial pressure of hydrogen within the package at or shortly after manufacture is at or near zero. Even after being placed in service, the devolution of hydrogen is very low, with partial pressures being of the order of 10−8 or atmospheres. Nevertheless, this hydrogen gas may be detrimental to the performance of the electronic device. For example, gallium arsenide (GaAs) or indium phosphide (InP) integrated circuits may degrade in performance when exposed to hydrogen levels as low as 100 parts per million (ppm).
In those cases where the package structure is hermetic, the outgassed hydrogen gradually accumulates inside the confined interior of the package. Even when the hydrogen is outgassed through a vent, these vents operate by diffusion, which is a temperature dependent process, although the electronic packages are designed to operate at temperatures in the range of about −25° C.–125° C. (−13° F. –260° F.), so that venting is also a slow process. The accumulated hydrogen may eventually cause degradation of the electronic device. In some instances, such degradation is not of great concern because the packaged electronic device can be readily replaced when it shows signs of degradation. In other cases, such as where the electronic device is part of a space satellite system that requires high reliability and is not readily accessible for service, the hydrogen-induced degradation is of great concern.
Several techniques have been developed to reduce the incidence of hydrogen degradation. In one, the assembled package and its electronic device are outgassed in a vacuum at elevated temperature for extended periods prior to service. This approach, while operable to some degree, may not be satisfactory because the concentration of hydrogen in the package materials and platings is unknown. In such cases, even thousands of hours of pre-service vacuum outgassing may not be sufficient. It is apparent that some hydrogen gas remains in the packaging material even after outgassing. In another approach, the package is not hermetically sealed so that the evolved hydrogen can leak from the package. A non-hermetic package is generally not satisfactory, because environmental contaminants such as water vapor can leak into the package and lead to degradation of the electronic device.
There remains a need for a satisfactory approach to the avoidance of hydrogen degradation of packaged electronic devices. Thus, it is desirable to minimize the amount of hydrogen to the maximum extent possible, while also providing a means for removal of any remaining hydrogen that otherwise may be entrapped within the hermitically sealed package. This can best be accomplished by reducing or eliminating sources of hydrogen. The present invention fulfills this need, and further provides related advantages.